Email should always have an undo button. It is far too easy to send out an angry or ill-advised email, particularly when our natural inclination to advocate for and protect our clients gets the better of our common sense and decency. Fortunately, if you are a Gmail user, the undo button is built-in.
In a post by Dan Pinnington on his SlawTips blog, he describes how to use Gmail's "Undo Send" feature:
To enable Undo Send, go into your Gmail Settings and hit the Labs tab. Scroll down and enable Undo Send, then hit the Save button (and while you are in there, you can peruse some of the other cool features that the people at Google Labs have created). Next, go to the General tab and set your “cancellation period” in the Undo Send section. Hit the Save button and exit Settings.
Undo Send lets you delay sending by 5, 10, 20, or 30 seconds. Which you choose depends on how quickly you can come to the realization that you were a jerk when your wrote the message. For most of us, we should pick the longest (30 second) option.